1. Field of the Invention
The present invention generally relates to a novel lighting circuit for a vehicular discharge lamp. More particularly, this invention pertains to a novel lighting circuit for a vehicular discharge lamp, which can detect an abnormal status of the circuit for protection against possible damage and can prevent a user from receiving an electric shock when replacing the lamp.
2. Description of the Related Art
In a lighting circuit for an incandescent lamp, a widely-used light source for vehicular headlamps, a relay coil is connected via a lighting switch between the terminals of a battery, and the incandescent lamp is connected via a relay contact between the battery terminals.
When an abnormality occurs in the operational status of such a lighting circuit for an incandescent lamp due to expiration of the service life of the lamp or the like, the worst possible consequence would be disconnection of the filament. No particular protection circuit is typically provided.
Metal halide lamps have recently been receiving greater attention as a new light source replacing the incandescent lamp. To use this metal halide lamp as a vehicular headlamp, it is necessary to instantaneously light or relight the lamp. The activation voltage for this event is very high (about 10 to 20 kV).
When setting the lighting switch on does not light the lamp because of occurrence of some abnormality in the lamp originating from the expired service life of the lamp or the like, a high voltage keeps appearing between the output terminals of the lighting circuit. As a result, a dielectric breakdown may occur in contact terminals provided in the socket where the lamp is installed, thus causing fire. There is also a possibility that a user will receive an electric shock, replacing the bad lamp without knowing a high voltage applied to the lamp.
Further, when the output current or output voltage of a DC voltage booster circuit, one component of the light circuit, the circuit elements may be damaged or the lighting circuit may malfunction due to the damage, thus generating a high voltage (which will cause troubles brought about by electromagnetic interference) and damaging the lamp.